1. Field of Invention:
This invention relates generally to stereoscopic (3-D) television systems, and more particularly to a 3-D converter assembly usable in conjunction with a standard 2-D closed circuit television system in which a video camcorder functions to record moving images of a scene and to play back the recording through a TV monitor on whose screen the moving images are exhibited. The assembly in the recording mode causes the system alternately to record left and right images of the scene to produce a 3-D recording which during the playback mode is fed to the TV monitor whose screen exhibits these alternating images which are seen through an electro-optical viewer by an observer to whose left and right eyes are alternately presented the corresponding screen images to provide stereoscopic vision.
2. Status of Prior Art:
As with cinematic motion pictures, a TV system, whether of the 2-D or 3-D type, exploits the phenomenon of visual persistence. The human eye is not instantly responsive to visual stimulus, nor does the sensation of vision disappear immediately upon removal of the stimulus. Because of visual persistence, when images of related but slightly different scenes are received in rapid succession, there is conveyed to the observer the sensation of apparently continuous motion.
If the rate at which the different image is presented to the viewer is relatively slow, the effect of motion is accompanied by flicker. When the image projection rates are sufficiently high, flicker disappears, but the frequency at which this happens depends on the brightness of the field being viewed. Hence the more luminous the field, the higher the flicker or fusion frequency.
The three-dimensional image of an object is observed in a slightly different manner by the pupils of the left and right eyes of the viewer, for these pupils are not coincident. The dissimilarity existing between two simultaneously seen images, each of a different angular perspective, enables the brain to fuse these images so as to create a three-dimensional mental image of the object. The perception of depth thereby gained is referred to as stereoscopic vision.
The reason a conventional TV system is sometimes referred to as cyclopean is that images are viewed by a video camera whose monocular lens function in a manner equivalent to that of a single human eye; hence the 2-D image picked up by this camera and presented on the screen of a TV monitor is lacking in depth. The present invention relates to 3-D or stereoscopic TV systems, and for this purpose the system must be capable of presenting to the left and right eyes of the viewer slightly different images of a scene.
Prior art 3-D television systems, in order to present a viewer with stereoscopic (3-D) TV images, makes use of various methods of image separation and recombination, including electromechanical and electro-optical shutters for this purpose.
Thus, the 1975 U.S. Pat. No. to Roese, 3,903,358, discloses a stereoscopic TV system which functions to present alternate left and right images to a single video camera whose output is fed to a TV monitor. The viewer is provided with eyeglasses having left and right PLZT electro-optic light valves, the opening and closing of which are controlled in synchronism with the left and right image presentations by the TV monitor. In this way, the viewer alternately sees the left and right images and gains depth perception.
As described by Roese and Khalafalia in "Stereoscopic Viewing with PLZT Ceramics," Ferroelectrics, Vol. 10 (1976), pp 47-51, these eyeglasses comprise a pair of PLZT (lead lanthanum zirconate titanate) devices that form the eye pieces for the left and right eyes of the wearer. Each device includes an optically cemented assembly of anti-reflective coated glass, a front polarizer, a PLZT ceramic wafer and a rear polarizer. When a high voltage (i.e., 400 V) is applied to the device, it changes its birefringence and thereby the direction of polarization of light incident thereto. Changing the polarization direction of light by the PLZT device causes either the transmission or extinction of the light to provide a shuttering action.
In the Roese '358 patent, in order to effect a left and right eye presentation of the image, use is made of the interlaced pattern of the two scanning fields of a standard TV frame. In a conventional TV system, the scanning lines which form the second field fall between the lines forming the first field and are interlaced therewith, the first and second fields creating a single frame. In the Roese 3-D system, the first field provides the left eye image of a scene, while the second field provides a slightly different right eye image of the same scene.
In the United States, TV standards dictate 30 frames per second, with 525 lines per frame and 262.5 lines per field. Other countries have higher resolution standards. Thus, Great Britain requires 625 lines per frame and 50 fields per second, while in France there are 819 lines per frame and 50 fields per second. The present invention is applicable to any 2-D TV system using alternate fields for the left and right eyes regardless of the national standards imposed on the system.
Also of background interest is the 1973 U.S. Pat. No. to Kratomi, 3,737,567, which discloses a 3-D TV system making use of eyeglasses having left and right liquid-crystal shutters that are alternately opened by a sync signal derived from left and right eye TV images. A similar system is disclosed in the 1971 Hope U.S. Pat. No. 3,621, 127.
The Hope patent as well as the 1984 U.S. Pat. No. 4,424,529 to Roese seek to avoid wired connections between the shuttering glasses and the control unit therefore, and for this purpose the control signals are transmitted. A similar provision to avoid wired connections to the optical shuttering elements is disclosed in the 1981 Jurisson et al. U.S. Pat. No. 4,286,286.
The Jurisson et al. patent is of particular interest. While this patent discloses a pair of video cameras for picking up left and right images of a scene, rather than a single camera for this purpose as in the Roese patent, the Jurisson et al. system is capable of recording the stereoscopic video signals and playing back the recording of the alternate output of the two cameras on a TV monitor which is viewed by shuttered eyeglasses.
U.S. Pat. No. 4,523,226 to Lipton et al. (1985) and the Ikushima et al. U.S. Pat. No. 4,393,400 (1983) are also of interest, for these patents seek to eliminate flicker in a 3-D television system. In Lipton et al., the arrangement is such that each eye sees 60 fields per second, rather than 30 fields per second as in the Roese system. Since 60 fields per second is well above the critical fusion frequency, flicker which may appear in the 3D system of the Roese type is not present. The present invention is also applicable to a flicker-free system of the Lipton et al. type.